The debate concerning the law, ethics and policy of autonomous weapons systems (AWS) remains at an early stage, but one of consistent emergent themes is that of uncertainty. Uncertainty presents itself as a problem in several different registers. First, there is the conceptual uncertainty surrounding how to define and debate the nature of autonomy in AWS. Second, the current debate about AWS is characterized by a temporal uncertainty concerning the timeline for the expected development of these systems and the pathways over which ‘autonomization’ of weapons systems’ functioning may be expected to take place. The final register of uncertainty that appears in the debate is that of the unpredictability and uncertainty of how robotic and computer systems will behave in unstructured and rapidly changing environments. Against this backdrop, we argue that a plausible and relevant set of international legal principles for addressing these problems of uncertainty derive from international environmental law, which, as a matter of existing law, could be considered under article 36 of the 1977 Additional Protocol I to the 1949 Geneva Conventions.